The present invention concerns a new scheme for optical lithography that allows extension of optical lithography beyond the conventional resolution limits imposed by the wavelength of the exposing light.
The resolution of conventional optical lithography schemes is mainly limited by the wavelength of the light used for the transfer of a mask pattern onto a resist. The wavelength of the exposing radiation is a main determinant of pattern resolution W, given by the Rayleigh equation W=kjxcex/NA, where xcex is the wavelength of the exposing light, NA is the numerical aperture of the optical lithography tool, and kj is a constant for a specific lithography process. In other words, the resolution W is proportional to the wavelength xcex of the exposing light. Cutting edge production today creates features that are 250 nm wide using 248 nm illumination. Currently, the implementing schemes based on light are the bottleneck when trying to obtain structures of a feature size below 200 nm.State-of-the-art optical lithography system for making current DRAMs, for example, are quite expensive. Alternative processes become attractive when moving on to smaller feature sizes, but the required investments are huge. Thus techniques that maintain compatibility with much of the existing processes are inherently valuable.
Trends in both integrated circuit and flat-panel display manufacturing technologies require improvements in small scale lithography. In these and other fields, there is an increasing demand for a cost-effective lithographic technique that can produce large fields (to approximately 45 cm diagonal for displays) of nanoscale structures. The semiconductor industry road map calls for leading-edge manufacturing at 180 nm in the year 2001 and 70 nm in the year 2011.
One well known form of optical lithography is the so-called hard contact lithography, where the mask is moved directly into contact with the substrate targeted for patterning. Features on a mask, comprising alternatively translucent and opaque regions in a well-defined pattern, are printed into photo resist in a 1:1 relation to their area on the source. Hard contact lithography can, in principal, make structures with sizes below the wavelength of illumination. But the contact used to place the mask on the substrate compromises the integrity of the process as the possibility of confounding material on the surface of the mask and mask damage greatly limit (compared to projection lithography) the useful number of prints it can form. Cost is particularly worrisome as the feature scale shrinks and the expense of mask fabrication skyrockets with the increase in the density of its features. Contact masks are also generally much more expensive than those used in optical projection lithography since the critical dimensions in the former need to be smaller than those in the latter, for equivalent resolution, by the reduction factor used in a projection system. Dust particles and other physical impediments to the substrate are catastrophic in hard contact lithography as they lift the mask away from the surface, blurring the pattern. Such defects appear over an area much larger than the obscuring particle because the mask is unable to conform around their presence; this problem is compounded as the feature scale shrinks such that even a 200 nm particles can be harmful. In addition, resist can get stuck to the mask. Hard contact lithography has thus not found a significant role in manufacturing of small scale integrated circuits.
There are many approaches known, that improve conventional lithography systems in that filters, projection lens, or appropriately modified masks are employed. These approaches get more and more complicated and expensive with reducing feature scale. One example here is the so-called optical projection lithography. The optical lithography based on projection is undoubtedly the most successful and widely employed means of making features down to xcx9c200 nm. Here, a pattern of intensity variations in the far field results when light is shown through a mask like that used in contact lithography. The light propagates through air and is focused by a lens to form an image of the desired pattern on a resist covered substrate, often demagnified by a factor of 5-10 from its size on the mask. Projection lithography is largely limited to features sizes at, and larger than, the wavelength xcex of light, however. Its implementation becomes increasingly difficult, in addition, as the scale shrinks towards, and below, 200 nm, where very complicated systems of lenses and materials are required to carry out existing and proposed schemes. The area over which uniform illumination can be achieved is particularly problematic. The maximum current field sizes in the best 248 nm exposure tools is now only xcx9c20xc3x9720 mm. The useful area of exposure will continue to shrink dramatically with the wavelength of illumination, principally because of the materials and engineering challenges in forming uniform exposures through complex lenses based on silicates.
It is then generally a disadvantage of most of these approaches that they are getting more and more complicated and expensive when trying to obtain smaller feature sizes. Furthermore, there is a tradeoff between maximum resolution, depth of focus and achievable field image which comes from the use of a lens to focus the light.
The resolution of standard photolithography systems can be increased and feature size decreased by using masks that manipulate the phase (referred to as phase masks, phase shifting masks or PSMs) instead of the amplitude of the light used for exposure. Two examples of phase shift-based approaches are described by D.M. Tennant et al. in xe2x80x9cPhase Grating Masks for Photonic Integrated Circuits Fabricated by E-Beam Writing and Dry Etching: Challenges to Commercial Applicationsxe2x80x9d, Microelectronic Engineering, Vol. 27, 1995, pp. 427-434, and by J. A. Rogers et al. in xe2x80x9cUsing an elastomeric phase mask for sub-100 nm photolithography in the optical near fieldxe2x80x9d, Appl. Phys. Lett., No. 70, Vol. 20, May 19, 1997, pp. 2658-2660.
Tennant et al. propose the use of hard contact masks, whereas Rogers et al. favor elastomeric masks 10 (see FIG. 1) for the formation of high density sub-wavelength features 17. In these methods the pattern on the mask results in interference in the illumination arising in the near field from contact between a photo resist 11 and the structured mask 10. Light passes everywhere through the mask 10 which is completely translucent but has a pattern of surface reliefs 14 that vary in a well-defined manner. Light traveling through such a structured mask 10 experiences a comparatively longer or shorter path depending on the place of its exit. This change in the effective path length through the structured mask 10 contributes to phase (and only phase) differences in the propagating light. These phase differences result in sub-wavelength nodes in intensity of the exposing radiation at the surface of the resist 11. If these masks 10 are designed and made appropriately, there are nodes at the mask/resist interface 15 with a relative minimum in intensity.
Rogers et al. showed that using a phase approach with an elastomeric mask allowed them to make sub-wavelength features 18 in a photo resist layer 11 while avoiding the problems associated with brittle contact masks (like Tennant et al.), as illustrated in FIG. 1. These features 18 can then be transferred into a substrate 16 by means of dry etching, or wet chemical dissolution of the substrate, as is well known in the art. The features 17 formed in the substrate have about the same lateral dimensions as the features 18 formed in the photo resist 11. The problem with the aforementioned approaches to lithography based on phase shifting of light through a mask 10 is that, while small features 17 (sub-wavelength) can be generated, these features 17 are constrained to a one dimensional geometry (lines) or low density on the substrate 16. Further, the shape of the structure 18 in resist is limited. In the paper of Rogers et al., the authors show that the phase shift in the light intensity provides structures 18 in resist 11 that are related to the derivative of the topology in the phase mask 10, i.e. each wall in the pattern of surface reliefs 14 in the phase mask 10 gives a relative minimum in the light intensity at the surface 15 of the resist 11. The width of this node is narrow but fixed so that only a very limited variation in the range of the lateral sizes of these features 18 and 17 is possible.
Making dots, squares or generally filled structures of arbitrary shape is not obviously possible in a single step by this technique. Moreover, in order for the phase shift to be present at all, the height of the surface reliefs 14 in the phase mask 10 must closely match the wavelength of the exposing light 13. This requirement of the phase shift approach means that the structures in a phase mask 10 are constrained to becoming increasingly anisotropic as their dimensions shrink, a significant problem in the formation of such features in elastomeric materials. It is another problem of these kind of approaches that there always are twin structures exposed in the resist, because each xe2x80x98legxe2x80x99 14 of the phase mask 10 generates a pair of nodes of low intensity at its edges.
The problem of mask fabrication described above remains, of course, as well as the susceptibility of the process to defects and damage. The use of an organic polymer to form the mask allows their convenient formation by a variety of techniques, perhaps most notably by replication of the mask from a master. Many polymeric masks can be cast on a single master with no evident wear of the latter since the process exerts no, or very little stress, on the substrate. Replication of masks avoids some of the problems associated with the costs of their use in contact lithographies for the formation of high density, small structures: the replica can be made so cheaply as to allow its use only once prior to its disposal. Not all structures are possible in many of the most convenient polymers, such as the elastomeric poly(dimethylsiloxane) used by Rogers et al., for these applications, however. The paper xe2x80x9cStability of Molded Polydimethylsiloxane Microstructuresxe2x80x9d by Delamarche et al. (Advanced Materials 1997, 9, p 741) showed that many features in ordinary elastomers collapse and tend toward increasingly poor definition and their anisotropy increases and their feature scale decreases, respectively.
There are proposals concerning other approaches and schemes by means of which the resolution of optical lithography systems can be somewhat extended to smaller feature sizes. An example is given in an article by H. Fukuda et al. with title xe2x80x9cCan synthetic aperture techniques be applied to optical lithography?xe2x80x9d, published in J. Vac. Sci. Technol. B, Microelectron. Nanometer Struct. (USA), Vol. 14, No. 6, Nov. Dec. 1996, p. 4162-4166. This article discusses the theoretical feasibility of applying optical aperture synthesis to lithography. A technique involving the insertion of three phase gratings into a conventional projection system is described. While this approach paraxially yields imaging with doubled spatial bandwidth, aberrations introduced by the gratings are shown to be a serious limitation. Image simulations demonstrated that for very restricted pattern types, resolution down to 0.1 xcexcm is theoretically achievable.
Using expensive optics and existing laser sources and photoresists, interferometric lithography can be extended to well beyond the horizons of the current industry road maps, as described by Ch. Xiaolan et al. in xe2x80x9cMultiple exposure interferometric lithographyxe2x80x94a novel approach to nanometer structuresxe2x80x9d, Conference Proceedingsxe2x80x94Lasers and Electro-Optics Society Annual Meeting 1996, p. 390-391.
More exotic schemes for sub-200 nm lithography are also being considered. X-ray, extreme UV and projected beams of electrons are all now the object of active research for manufacturing application. The problems with these techniques range from the difficulties of mask fabrication, the implementation of practical beam formation techniques, the need for novel resist materials that allow a functional and sensitive use of the beam intensity, practical problems in forming and stabilizing the beam and the ever present limitations of cost and complexity. It is an object of the present invention to provide a new optical photolithography scheme that allows extension of the use of optical lithography systems to feature sizes well below 350 nm, and in particular to feature sizes in the range between xcex/2 and xcex/5, using existing light sources.
The above objects have been accomplished by providing a parallel optical lithography system (referred to as light coupling structure) based on sets of light coupling portions and light blocking portions. The light coupling portions guide the exposing light towards a resist to be exposed and are designed to be brought into conformal contact with the resist such that the refractive index of the resist is selectively matched by the light coupling portions at predetermined locations on its surface. This selective matching of index by light coupling portions adjacent to index blocking structures allows light to be selectively and deterministically guided and coupled into defined regions of the resist. The lateral shape and size of the protruding elements defines 1:1 the lateral size and shape of small features to be exposed in a resist.
It is herein taught how to form and use light coupling structures that guide light to a surface that allow a new form of contact lithography for the parallel fabrication of sub wavelength features having arbitrary shape and high density on a substrate. We depend on the formation of these light couplers by direct contact of a substrate with a translucent mask (herein referred to as light coupling structure) that act to direct light onto the surface where the pattern forms. We use methods (described below) that avoid the use of reference waves and thus suppress interference effects common to the lithographies based on phase shifting light (phase shift mask approaches).
It is an important advantage of the inventive approach that no imaging optics are required. Large areas can be structured in single exposures since the present approach is inherently parallel as all features are exposed in the resist simultaneously providing a high throughput. The inventive scheme can be used for large field images and is thus well suited for the manufacturing of displays as well as the batch processing of semiconductor chips, such as DRAMs and so forth, and wherever else the formation of high density small structures is required. The present invention is also well suited for the formation of micro-mechanical structures.
The elastomeric light coupling structures can be easily replicated from a master copy, and each replica can be used many times.
It is an advantage of the present invention that compatibility with existing resist and processing technologies is maintained.
It is a further advantage of the present invention that one can make use of the huge amount of experience in the design and handling of resists, because these resists are continued to be used.
It is a further advantage, that multiple wavelengths can be used without adjustment of the light coupling structure, since lenses are not used.
It is another advantage of the present invention that the time of exposure is shorter than in case of conventional masks because lens and other means do not block, absorb or scatter the light unproductively.
It is further the advantage of the present invention that lithographic schemes implemented by its use are simple.